TMZ spoke to a rep for Bruce -- the guy who rocks the overalls and the American flag bandana -- who tells us Tyler was having "heart troubles" back in January ... and had to be rushed to the vet for emergency medical care.

We're told things got so bad "the dog almost died."

Luckily for Tyler, doctors were able to treat the illness ... and he's been resting at home and taking heart meds ever since.

The rep tells us Bruce is unsure if Tyler will be able to join him on the boat next season ... saying Bruce is "hopeful...but still not sure."